Endoscopes are well known and widely used in the medical field. In general terms, endoscopes are grouped into two general categories, rigid or flexible. Typically, rigid endoscopes comprise an elongated shaft housing an optical system which conveys light energy from a distal end of the shaft to an ocular positioned at the proximal end of the shaft. At times, medical practitioners directly view the inside of a body cavity via the endoscope ocular. In recent years, however, a hand-held camera is detachably affixed to the endoscope to generate and transmit images representative of the endoscope field of view to a surgical display or monitor. Typically, the camera (e.g., a “camera head”) includes a solid-state image sensor which converts detected light energy into electronic signals.
Additionally, some endoscopes are provided with integrated imagers which obviate the need for a separate camera head. Endoscopes with integrated imagers (both rigid and flexible) are generally referred to as “video endoscopes”. Video endoscopes, as are those endoscopes with detachable camera heads, are typically either wired or wirelessly in communication with a Camera Control Unit (“CCU”). CCU's receive, from the camera head or video endoscope, the electronic signals representative of the endoscope field of view.
As with both camera heads and video endoscopes, the electrical distance between the image sensor and any associated supporting timing generator can vary greatly, depending upon the overall design of the video system. Therefore, the length of certain signal transmission lines extending from the image sensor or from the CCU, may vary depending on the type of video endoscope and/or camera head utilized. As signal transmission lines increase in length, certain timing signals may be distorted due to limitations of the signal conductor and/or by propagation delay. This undesired timing signal corruption may, in turn, produce inaccuracies and/or corruption to the resulting displayed video.
The degree of image signal deterioration/corruption caused by signal transmission line mechanics of design and/or overall signal transmission line propagation delay is proportionally related to the length of the transmission line. There are many types of endoscopes for respective applications and the lengths of the cables can vary from approximately half a meter to several meters in length (i.e., the endoscope insertion sections as well as cables for connecting the endoscope to the CCU).
One approach to deal with this problem is to have a dedicated CCU for each respective endoscope. However, this approach is just not economical nor is it practical in terms of space-saving or time if an endoscope needs to be changed during a medical procedure. Instead, a single CCU is commonly used to support a plurality of endoscopes. As cable length increases, the amplitude, DC level and phase of image sensor timing signals, which are supplied from the CCU to the image sensor via the cable, as well as amplitude and phase of the image signal transmitted from the image sensor to the CCU also via the cable, vary with different types of endoscopes. This timing and image signal variance may become so great that it becomes extremely difficult, if not impossible, to process the image signal for presentation to the display.
In order to solve these problems, some video systems have provided a cable/head delay circuit that directly uses a signal that makes a cable/head round trip. These cable/head delay circuits are used for adjusting the timing and/or image signal(s) depending on the type of the endoscope used. However, these cable/head delay circuits, are typically complicated in design and configuration, and exhibit reduced reliability. On the other hand, if a cable/head delay circuit is housed within the endoscope(s), a plurality of design configurations must be provided for various types of endoscopes, thus entailing an increase in component numbers required in the system. Additionally, such circuitry takes up valuable space and energy in the endoscope, and adds to the overall weight of the device, which is highly undesirable for a surgeon that has to hold the endoscope for many hours during a surgical procedure.
U.S. Pat. No. 4,860,095, Kimura et al. describes a system that utilizes an analog cable/head delay circuit. Kimura et al. teaches that image sensor driving signals are generated in the control and video processing circuitry and passed to the image sensor (CCD) by means of the cable. To deal with the decay in these signals that would otherwise occur in the cable, circuitry is provided to appropriately adjust the amplitude of the driving signals before the same are sent over the cable. To adjust for the delay of these signals, a Phase-Locked Loop (PLL) is provided to continuously maintain a constant phase relationship between the driving signals, and the composite signal. In other words, Kimura et al. teaches that timing is achieved by using a signal that has made the cable/head round trip in a PLL. However, this approach suffers from well-known instability that plagues classic PLL's. This design is also extremely complex because of the large number of signal lines in the cable and the level of circuitry required for transmitting the driving signals to the CCD.
A major problem with prior art approaches has been the introduction of an analog delay line that introduces jitter (timing noise) to the signal. This jitter is unavoidably translated to voltage noise during the analog-to-digital conversion. Voltage noise, in turn, is seen as visible noise in the viewed image resulting in a corrupted image.